Radio apparatus



March 29, 1938. w, EA 2,112,382

RADIO APPARATUS Filed April 15, 1936 CUEflENT- SECDNDABY FREQUENCY-F @emwe W 62:42.- g MS Patented Mar. 29, 1938 UNITED STATES ATENT OFFIQE Claims.

The present invention relates in general to radio apparatus, and is more specifically concerned with improved means whereby contracted selective tuning for receiving distant stations may 5 be attained, or, if desired, expanded high fidelity tuning may be quickly and easily effected for local station reception.

In the past, high fidelity and selective conditions of tuning have been accomplished in a variety of ways. In one such arrangement, the mutual inductance between the tuned primary and tuned secondary of the intermediate frequency coupling transformers was varied mechanically, that is, by a manual shifting of one winding of the coupling transformer relative to the other.

In other cases, these conditions of tuning have been accomplished electrically by means of switches connected into suitable circuits, the manipulation of these switches acting to connect condensers, inductance coils, and the like, into the circuits, whereby the resonant conditions might be varied or detuning of the circuits accomplished.

Of the two types of tuning, mechanical and electrical, the latter has proved more satisfactory from the standpoint of the user of the apparatus. The electrical arrangement, however, from the standpoint of cost has been relatively expensive to manufacture.

3 The present invention proposes to provide an improved apparatus wherein the mutual inductance between the primary and secondary windings of the intermediate frequency coupling transformers may be quickly and easily varied to obtain either a selective tuning or high fidelity tuning, as desired.

It is a further object of the invention to provide an improved coupling transformer for accomplishing the foregoing purpose.

Another object of the present invention is to provide a coupling transformer wherein the mutual inductance between its primary and second ary windings may be varied electrically rather than mechanically.

Still another object is to provide radio apparatus wherein selective and high fidelity tuning may be accomplished without the necessity of having to vary the effective number of turns of the coupled reactances of the coupled tuned circuits.

Still another object is to provide a novel arrangement for varying the mutual inductance between the coupled reactances of coupled tuned circuits, wherein electrical means are provided for selectively increasing and decreasing the ma netic coupling between the reactances.

In accordance with the general features of the present invention, it is proposed to provide an intermediate frequency transformer having a primary and secondary winding in loosely coupled relation. A third or tertiary winding is closely 5 coupled. in relation to the primary winding on the transformer. The tertiary winding is provided with a tap between its ends, this tap being connected to one end of the secondary winding of the transformer. By connecting the transformer 10 output circuit to the other end of the secondary winding and one or the other of the ends of the tertiary winding, current may be caused to flow through one part or the other of the tertiary winding in such direction as to produce a magnet 15 flux which. will either increase or decrease the magnetic linkage between the primary and secondary windings of the transformer. This change in the mutual inductance or magnetic linkage between the primary and secondary wind- 0 ings on the transformer may be taken advantage of to secure selective and high fidelity tuning, as desired.

In other words, with the bucking portion of the tertiary winding in the circuit, the primary and 5 secondary windings would be so spaced and adjusted as to provide substantially critical coupling, and with the boosting portion of the ter tiary winding in the circuit, provide more than critical coupling. 30

Other objects and features of the invention will more fully appear from the following detailed description, taken in connection with the accompanying drawing, which illustrates a single embodiment thereof, and in which: 5

Figure 1 is a view illustrating a normal assembly of a superheterodyne receiver;

Figure 2 shows schematically the circuit detail of one stage of the intermediate frequency amplifier; 40

Figure 3 shows schematically the circuit detail of part of the intermediate frequency amplifier embodying the features of the present invention;

Figure l illustrates a coupling transformer constructed in accordance with my invention; 45 and Figure 5 is a graphical representation of the resonant curves attainable in the coupling arrangement of the present invention.

As shown on the drawing: 50

Figure 1 of the drawing shows an ordinary superheterodyne receiver which comprises a tuned input circuit A to which a local oscillator B is connected. The combined action of the oscillator and tuned receiving circuit produces a 55 signal modulated wave having a frequency lower than that of the incoming signal. This lower frequency or intermediate frequency is supplied to an intermediate frequency amplifier shown at C. From this amplifier, the signal ordinarily is supplied to an audio-frequency amplifier D which is connected to the loud-speaker E.

In order that the present invention may be more clearly understood, and since the invention is particularly concerned with the intermediate frequency amplifier arrangement, a portion of the intermediate frequency amplifier circuit is shown in Figure 2.

The signal modulated intermediate frequency current is supplied to the input circuit of vacuum tube to, which in this case, for simplicity, is shown as a three-electrode type. However, it is to be understood that the present invention does not require any specific kind of tube, as it will function equally well with other types of amplifying tubes than the one shown.

The output circuit of tube It! and the input circuit of tube H are, in the usual arrangement, magnetically coupled through an intermediate frequency coupling transformer, generally inclicatcd at l2. This transformer comprises a primary winding 93 and a secondary winding M. Connected across the primary winding is an adjustable condenser l5, and across the secondary winding an adjustable condenser E5. The two circuits |3l5 and i l-46 are respectively tuned by the adjustment of the condensers to resonate at a frequency chosen for the intermediate frequency. The primary and secondary windings, in the usual arrangement, are of fixed values of inductance, and the degree of their magnetic coupling is of a fixed amount, since the windings are fixedly mounted at some specified distance apart. The tuning of these two circuits is usually taken care of when the set is assembled at the factory, and having once been adjusted, the adjustment is not changed during the operation of the set. In Figure 3, there is shown a circuit detail. of the intermediate frequency amplifier (the amount shown inside the dotted rectangle in Figure 2), with the features of the present invention incorporated therein. Although the improved arrangement is described as being applied to only one intermediate frequency coupling transformer, it is to be understood that where a plurality of stages of amplification are present, the invention may be applied to more than one coupling connection between the successive amplifying tubes.

As shown in Figure 3, the present invention embodies an arrangement which includes an additional or tertiary winding N. This winding is closely coupled relative to the primary winding 13 and is provided with an inter-end tap which is connected as by a conductor It to one end of the secondary winding M.

The ends of the tertiary winding are brought out to switch contacts l9 and 20 which are selectively engageable by a movable contact member 2| of a switch. This contact member is con nected as by a conductor 22 to one side of the output circuit of the transformer, the condenser l5 being connected between the conductor 22 and the other end of the secondary winding I in By manipulation of the switch member 2! into engagement with either contact is or contact 20, the current of the output circuit may be caused to flow through portions of the tertiary winding, in opposite directions. It will therefore be evident that the passage of current through the tertiary in opposite directions will cause either a bucking or a boosting of the interlinking magnetic flux between the primary and second.- ary coils l3 and i4, and consequently vary the mutual inductance or coefficient of coupling between the primary and secondary windings.

For selective tuning, the primary and secondary coils and the bucking portion of the tertiary winding would be so positioned and proportioned as to give substantially critical coupling, as shown by the full line curve 23 in Figure 5.

Such a selectivity curve tends greatly to eliminate interference between two stations transmitting on nearly the same frequency, and such tuning is advantageous, especially when long distance reception is desired.

Such high selectivity is not, however, always desirable, and there are times when it is desirable to diminish the sharpness of the resonant curve and secure an expanded or high fidelity tuning, as shown by the dotted line curve 24 of Figure 5. This type of curve is particularly desirable in local reception.

There have been many schemes advocated for accomplishing the change from one curve to the other, that is, from selective tuning to high fidelity tuning, but in the main these schemes have been quite expensive and not entirely satisfactory from the standpoint of the user.

In the present invention, this change from one type of tuning to the other is accomplished in a very simple and inexpensive manner.

In order to obtain expanded or high fidelity tuning, in the present invention, the switch contact member 2! would be shifted into connection with the other end of the tertiary transformer, under which condition the magnetic flux of the tertiary winding would be in reverse direction to the other connection wherein the switch member has engagement with contact 19. On the latter connection, the coil winding would be changed to boost or increase the magnetic coupling between the primary and secondary windings i3 and I 4. The primary winding and the boosting part or section of the tertiary winding would be so proportioned and arranged in relation to each other that, with the boosting portion in the circuit, greater than critical coupling would be attained. This will result in a curve such as curve 24.

It will be noted that during both bucking and boosting conditions, substantially the same number of turns will be retained in the secondary circuit.

In Figure 4, there is shown a coupling transformer embodying the principles of the present invention. This transformer comprises a core 25 which may be of wood when the coils are air coupled, and may be of iron when it is desired to provide a coupling having lower reluctance. This core is supported at one end on a plate 25 of suitable insulation material, such as porcelain. At the other end, the core is supported in a removable insert 2! which may be of any suitable insulation material, fiber being used in this case.

The entire transformer is supported within a tubular housing 28 which is open at one end and closed, as shown at 29, at its other end. The in sulating members 25 and 2'1? are in peripheral engagement with the housing, and the assembly is retained within the housing by means of a threaded stud 30 which is anchored at one end to the plate 26 and projects through an opening in the end 29 of the housing, the assembly being secured by nuts 3! and 32 threaded on the stud BER and disposed on opposite sides of the end 29.

The plate 26 also carries adjustable condensers 33 and 34 composed of a plurality of plates separated by suitable insulating material, such as mica. These plates may be moved. into more or less closely spaced relation by means of an adjusting screw 35 in the case of each condenser. Access to these screws is provided by suitable openings 36 in the end 29 of the housing.

Mounted on the core 25 is a secondary coil 31 and a primary coil 38, these coils being in loose coupled relationship. The tertiary winding 39 is disposed around the core within the primary coil, and is consequently closely coupled relative to the primary coil.

An end of the secondary coil is shown at 40 and the other end is brought out as shown at 4! and connected to a point in the tertiary coil which is between its ends 42 and 43. The ends of the primary coil are shown at 44 and 45.

One of the condensers, for example, condenser 35, is connected through a conductor 46 to one end of the secondary coil, in this case end 4!). The other connection to the condenser is shown at 41.

The other condenser, 36, is connected by conductors 48 and 49 to the respective ends 44 and of the primary coil.

The input circuit of the transformer would be connected to conductors 44 and 45. One side of the output circuit would be connected to conductor 40 of the secondary coil, and the other side of the output circuit would be connected to conductor 41 and to the movable contact of a switch which would be operable to engage selectively switch contacts respectively connected to conductors 42 and 43.

From the foregoing description, it will be apparent that the invention as described herein provides improved radio apparatus wherein the mutual inductance between the primary and secondary windings of the coupling transformer may be quickly and easily varied to obtain either a selective tuning or high fidelity tuning, as desired; in which the change from one type of tuning to the other type of tuning is accomplished electrically; and which embodies a novel construction in the coupling transformer, whereby selective and expanded high fidelity tuning may be readily and easily accomplished at low cost.

It is, of course, to be understood that although I have described in detail a single embodiment of my invention, the invention is not to be thus limited, but only insofar as defined by the scope and spirit of the appended claims.

I claim as my invention:

1. The combination with a pair of tuned cir cuits having main reactances in coupled relation, of means for varying the coefiicient of coupling between said reactances, said means comprising a common auxiliary inductance, and means for reversing the direction of magnetic flux in said inductance.

2. The combination with a pair of tuned circuits having main reactances in coupled relation, of means for varying the coeflicient of coupling between said reactances, said means comprising an auxiliary inductance connected in circuit with one of said reactors and in inductively coupled relation with the other of said reactors, and means for selectively varying the direction of flow of current in said inductance.

3. The combination with a pair of tuned circuits having main reactances in coupled relation, of means for varying the mutual inductance between said reactances, said means comprising an auxiliary reactance closely coupled with one of the main reactances and having a tap connection substantially intermediate its ends connected in circuit with the other of said reactances, and a switch for selectively completing said circuit through the auxiliary reactance from said tap to one end or the other of the auxiliary reactance.

4. A pair of resonate circuits including a pair of main coupled reactances, a two part auxiliary reactance in coupled relation with the main reactances, one of said parts being adapted to buck and the other part to boost the magnetic coupling between the main reactances, the bucking and boosting parts of the auxiliary reactance being so proportioned as to effect substantially critical coupling and greater than critical coupling, respectively, between the main reactances, and switch means selectively operable to connect said bucking and boosting parts.

5. A pair of resonate circuits including a pair of main reactances, a .two part auxiliary reactance for varying the mutual inductance between the main reactances, one of said parts arranged to decrease and the other to increase the magnetic coupling between the main reactances, said main reactances and said one part being so disposed and proportioned as to provide substantially critical coupling, and the main reactances and the other part being so disposed and proportioned as to provide greater than critical coupling, and a switch for selectively connecting one or the other of said parts.

ARTHUR W. SEAR. 

